The present invention relates to spark plugs for spark-ignition engines, and more particularly the invention relates to a spark plug having an improved resistance to carbon fouling.
It has been the tendency of the spark plugs known in the art to cause incomplete combustion due to the propagation of flame, thus causing a carbon deposition on the surface of the leg portion of the plug insulator and thereby causing insulation failure. This has also been confirmed by the fact that the degree of carbon fouling is increased as the ignition timing is advanced even if the other conditions are kept the same.
On the other hand, when the burning of the mixture in the combustion chamber becomes vigorous, the pressure in the combustion chamber is increased and at the same time the production of carbon within the combustion chamber is increased. This pressure rise in turn causes the burned gas to flow into the space of the spark plug and in this case there is the danger of causing the carbon produced in the combustion chamber to deposit on the surface of the insulator facing the space.